This invention relates to an improved support material based on aluminum or an aluminum alloy for use in offset printing plates. The invention also relates to a process for the manufacture of the support material.
Support materials for offset printing plates are provided, on one or both sides, with a radiation(photo-) sensitive layer (reproduction layer), either directly by the user or by the manufacturers of precoated printing plates. This layer permits the production of a printing image of an original by photomechanical means. Following the production of this printing form from the printing plate, the layer support carries the image areas which accept ink in the subsequent printing process and, simultaneously, there is formed, in the areas which are free from an image (non-image areas) in the subsequent printing process, the hydrophilic image background for the lithographic printing operation.
For the above reasons, the following requirements are demanded of a layer support for reproduction layers used in the manufacture of offset printing plates:
Those portions of the radiation-sensitive layer which have become comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue, and without the developer substantially attacking the support material. PA1 The support, which has been laid bare in the nonimage areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water rapidly and permanently during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink. PA1 The radiation-sensitive layer must exhibit an adequate degree of adhesion prior to irradiation (exposure), and those portions of the layer which print must exhibit adequate adhesion following irradiation. PA1 The support material should possess high mechanical strength, e.g., with respect to abrasion, and good chemical resistance, particularly to the action of alkaline media. PA1 Even without a hydrophilizing post-treatment the non-image areas of printing plates are free from "staining" after development. This shows that the oxide surface produced in accordance with this invention is clearly superior to an oxide layer of comparable weight, which has been produced in an electrolyte containing H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 or a mixture of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4. PA1 The resistance to alkali of the oxide produced is superior to the resistance to alkali of an oxide produced in an aqueous electrolyte containing H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 or a mixture of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4. PA1 The oxide layer weight obtained can attain the weight of an oxide layer produced in a H.sub.2 SO.sub.4 -containing electrolyte, and thus, in respect of layer thickness, is superior to the oxide produced in customary H.sub.3 PO.sub.4 -containing electrolytes. PA1 The oxide layer possesses good hydrophilic properties, so that a hydrophilizing posttreatment step, as known in the art of platemaking, can optionally be dispensed with. PA1 Of the light impinging upon the support, a high fraction of more than 5 is directly reflected, which manifests itself in a bright appearance of the support, and therefore the support coated with a photosensitive layer has a markedly better resolution than supports of comparable graining structure, which have been anodized in other electrolytes than those of the present invention. PA1 The very light non-image areas of the final printing form yield a strong contrast to the image areas, which is particularly advantageous in those cases, in which modern, optically working measuring instruments are used for measuring the proportions of image and non-image areas. PA1 The photosensitive layers adhere extremely well to the support surfaces and, as a result, the print runs obtained with these printing plates are markedly higher than the print runs produced with printing plates having supports of comparable type, but which have not been anodized according to the present invention. PA1 Mechanical abrasion is considerably lower than in the case of supports produced according to other methods, which have oxide layers of comparable weight. PA1 The good conductivity of the electrolyte and the electrode systems make it possible to operate with lower voltages. PA1 If, prior to graining, the aluminum is subjected to cleaning in an alkaline pickling agent, this pickling solution ca also be used for the anodizing treatment, provided it has a composition according to the present invention.
As the base material employed for layer supports of this type, aluminum is frequently used. It is superficially grained by means of known methods, such as dry brushing, slurry brushing, sandblasting, chemical and/or electrochemical treatment. Especially the electrochemically grained substrates are then subjected to an anodizing treatment, during which a thin oxide layer is built up, in order to improve the abrasion resistance. These anodic oxidation processes are usually performed in electrolytes such as H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.2 C.sub.2 O.sub.4, H.sub.3 BO.sub.3, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof. The oxide layers built up in these electrolytes or electrolyte mixtures are distinguished from one another by their structures, layer thicknesses and resistance to chemicals. Aqueous solutions of H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 are predominantly employed in the industrial production of offset printing plates. As far as electrolytes containing H.sub.2 SO.sub.4 are concerned, reference is made, for example, to European Patent No. 0,004,569 (=U.S. Pat. No. 4,211,619) and to the prior art publications mentioned therein.
Aluminum oxide layers produced in aqueous electrolytes containing H.sub.2 SO.sub.4 are amorphous and, in the case of offset printing plates, in general have a layer weight of about 0.5 to 10 g/m.sup.2, which corresponds to a layer thickness of about 0.15 to 3.0 .mu.m. When a support material, which has been anodically oxidized in this way, is used for offset printing plates, a disadvantage is presented by the relatively low resistance of oxide layers produced in H.sub.2 SO.sub.4 electrolytes to alkaline solutions. Solutions of this type are employed, to an increasing extent, for example, in the processing of presensitized offset printing plates, preferably in state-of-the-art developer solutions for irradiated negative-working or, in particular, positive-working radiation-sensitive layers. Furthermore, these aluminum oxide layers often tend to a more or less irreversible adsorption of substances from the applied reproduction layers, which may, for example, lead to a coloration of the oxide layers ("staining"). The supports anodized in this way are relatively dark and exhibit unfavorable values of mechanical abrasion.
It is also known to anodically oxidize aluminum in aqueous electrolytes containing oxyacids of phosphorus and optionally additional compounds, as described, for example, in German Offenlegungsschrift No. 32 06 470; this publication also gives a detailed evaluation of the prior art. The plates so produced look somewhat lighter than the plates anodized with sulfuric acid and they also have, for example, an improved resistance to abrasion and alkaline media, but they still do not have the desired silvery appearance and tend to halations.
Moreover, currentless alkaline treatments of aluminum oxide surfaces are, for example, disclosed in Patent of the German Democratic Republic No. 208 176, Japanese Patent Application Disclosure Nos. 57/177 497 and 56/051 388 and in German Offenlegungsschrift No. 32 19 922.
Japanese Patent Application Disclosure Nos. 57/085 998 and 57/085 996 describe a process for the anodization of solar heat energy absorbing plates, in which the electrolyte used contains, in addition to an alkali metal hydroxide, also an acid and sodium phosphate, a polyalcohol or a fluoride. Because of these additives, the pH of the electrolyte is too low for an application in the process of the present invention. The oxide layers obtainable with this electrolyte under technically appropriate conditions are consequently not as thick as the oxide layers which are advantageous for lithographic purposes. Besides, the addition of a fluoride increases the corrosive character of the electrolyte, which leads to an ugly gray appearance of the treated surface.
European Patent Application No. 0 048 988 describes a multi-stage process for coloring the surface of aluminum, in which streak patterns are formed. The material employed in this process has previously been anodized. In the second process stage--the coloring stage--an electrolyte is used, which contains alkali metal ions and additives which lead to the streak pattern. The additives mentioned comprise alkali metal phosphates or borates or alkaline earth metal compounds and an acid which serves to adjust the pH to a value below 5.0. Some alkaline electrolytes are known for producing a thin, but very dense electrically insulating barrier layer which prevents the formation of an oxide layer which is sufficiently thick for lithographic applications (see, for example, Wernick and Pinner: "The Surface Treatment and Finishing of Aluminium and its Alloys", Vol. 1, page 304 et seq., Robert Draper Ltd., Teddington 1972). This applies in the case of anodizing treatments in alkali metal phosphate solutions, which are described, e.g. in Japanese Patent Appln. Disclosure No. 54/031 047 and in German Offenlegungsschrift No. 28 42 396, in the case of electrolytes containing borates as the main constituents, as described in European Patent Application No 0 008 212, in Japanese Patent Appln. Disclosure No. 49/035239 and in British Pat. No. 1 243 741 and also in the case of electrolytes containing ammonium salts, as proposed by Japanese Patent Publication No. 9453/73.
In weakly alkaline solutions as described, for example, in Japanese Patent Appln. Disclosure No. 52/120 238 sufficiently thick oxide layers cannot be obtained, when technically appropriate voltages are applied during treatment times which are as short as required in modern continuously working processing units.
Japanese Patent Appln. Disclosure No. 53/011 843 recommends an electrolyte which may comprise an acidic or alkaline solution and contains a chloride. An aluminum material treated with this electrolyte has an unsightly, irregular gray surface, due to the corrosive action of the chloride ions.